1. Field of the Invention
The present invention relates to a new lettuce (Lactuca sativa) variety which exhibits resistance against downy mildew (Bremia lactucae Regel), currant-lettuce aphid (Nasonovia ribis-nigri), corky root (Sphingomonas suberifaciens), and lettuce mosaic virus (LMV).
2. Description of Related Art
All cultivated forms of lettuce belong to the highly polymorphic species, Lactuca sativa, which is grown for its edible head and leaves. As a crop, lettuces are grown commercially wherever environmental conditions permit the production of an economically viable yield.
Lactuca sativa is in the Cichoreae tribe of the Asteraceae (Compositae) family. Lettuce is related to chicory, sunflower, aster, dandelion, artichoke and chrysanthemum. Sativa is one of about 300 species in the genus Lactuca. 
Lettuce cultivars are susceptible to a number of diseases such as downy mildew, currant-lettuce aphid, corky root and lettuce mosaic virus (LMV). These diseases result in millions of dollars of lost lettuce crop throughout the world every year.
Downy mildew (Bremia lactucae Regel) is highly destructive of lettuce grown at relatively low temperature and high humidity. Downy mildew is caused by a fungus, Bremia lactucae Regel, which can be one of the following strains: NL1, NL2, NL4, NL5, NL6, NL7, NL10, NL12, NL13, NL14, NL15, NL16, B1:17, B1:21 and B1:23 (Van Ettekoven, K. et al., “Identification and denomination of ‘new’ races of Bremia lactucae,” In: Lebeda, A. and Kristkova, E (eds.), Eucarpia Leafy Vegetables, 1999, Palacky University, Olomouc, Czech Republic, pp. 171-175.)
Downy mildew causes pale, angular, yellow areas bounded by veins on the upper leaf surfaces. Sporulation occurs on the opposite surface of the leaves. The lesions eventually turn brown, and they may enlarge and coalesce. These symptoms typically occur first on the lower leaves of the lettuce, but under ideal conditions may move into the upper leaves of the head. When the fungus progresses to this degree, the head cannot be harvested. Less severe damage requires the removal of more leaves than usual, especially when the lettuce reaches its final destination.
Of the various species of aphids that feed on lettuce, the currant-lettuce aphid (Nasonovia ribis-nigri) is the most destructive species because it feeds both on the leaves of the lettuce as well as deep in the heart of the lettuce, making it difficult to control with conventional insecticides. The lettuce aphid feeds by sucking sap from the lettuce leaves. Although direct damage to the lettuce may be limited, its infestation has serious consequences because the presence of aphids makes lettuce unacceptable to consumers. Additionally, the lettuce aphid has a capacity to act as a vector for a number of viruses, such as gooseberry veinbanding virus, cauliflower mosaic virus, cucumber mosaic virus and lettuce mosaic virus.
Corky root (Sphingomonas suberifaciens) is a soilborne bacterium that is prevalent in most coastal lettuce growing areas. Corky root affects both leaf and head lettuce varieties. The disease typically is more severe when soil temperatures are warmer and in fields where lettuce is grown consecutively. High soil nitrate levels also increase disease severity. Early symptoms of corky root are yellow bands on tap and lateral roots of lettuce seedlings. The yellow areas gradually expand, taking on a greenish-brown color and developing cracks and rough areas on the surface of the root. As disease severity increases, the entire tap root may become brown, severely cracked, and nonfunctional; the feeder root system also may be reduced and damaged. At this point, roots are very brittle and easily break off when examined. Corky root also may cause internal discoloration of the root. When the root is severely diseased, aboveground symptoms consist of wilting during warm temperatures, stunting of plants, and general poor and uneven growth.
Lettuce mosaic virus (LMV) mainly infects lettuce seeds, which is the primary way that the virus is introduced to lettuce in the fields, but also can infect numerous crops and weeds, thereby creating reservoirs of the virus. LMV also can be vectored by aphids, which spread the virus within a lettuce field and introduce it into lettuce fields from infected weeds and crops outside the field.
Symptoms of lettuce mosaic virus vary greatly. Leaves of plants that are infected at a young stage are stunted, deformed, and (in some varieties) show a mosaic or mottling pattern. Such plants rarely grow to full size; head lettuce varieties infected early fail to form heads. Plants that are infected later in the growth cycle show a different set of symptoms. These plants may reach full size, but the older outer leaves turn yellow, twisted, and otherwise are deformed. On head lettuce, the wrapper leaves often will curve back away from the head and developing heads may be deformed. In some cases brown, necrotic flecks occur on the wrapper leaves.
There exists a need for an improved harvestable lettuce product which exhibits a combination of resistance against downy mildew (Bremia lactucae Regel), currant-lettuce aphid (Nasonovia ribis-nigri), corky root (Sphingomonas suberifaciens) and lettuce mosaic virus (LMV).